1. Field of the Invention
This invention relates to an image processing apparatus, an image processing method, and a computer-readable medium.
2. Description of the Related Art
An image processing apparatus is provided that forms an image by scanning and exposing a photosensitive member with a laser beam. Such an image forming apparatus deflects the laser beam by a rotating polygon mirror and scans the photosensitive member to perform exposure (exposure scanning) based on image data. A latent image formed by the exposure (exposure scanning) is developed to form an original image, which is transferred and fixed onto a recording medium such as paper. It is known that a color printer performs the above-described processing with a plurality of process color (e.g., yellow, magenta, cyan, and black) and superimposes the original images of each color to form a multiple color image on the recording medium.
In such an image forming apparatus, when the exposure is performed, due to skew of a scanning line (exposure scanning line) caused by plane tilt of the polygon mirror, the original image is skewed in a curved-line shape. On the other hand, Japanese Patent Application Laid-Open No. 2009-034865 discusses a technique for offsetting the skew of the formed original image by correcting (skew correction) the image data considering the skew of an exposure scanning line. Such skew correction is performed using correction information approximately representing the skew of the exposure scanning line in an area (also referred to as a “main-scanning width area”) of a main-scanning direction in which the latent image is formed by the exposure scanning.
The skew correction discussed in Japanese Patent Application Laid-Open No. 2009-034865 includes line shifting processing and smoothing processing. In the line shifting processing, a centroid of the image data is displaced by a pixel unit by shifting the image data in a sub-scanning direction. In the smoothing processing, a pixel value of the image data is changed according to a position in a main-scanning direction to displace the centroid of the image data by less than a pixel unit. When the line shifting processing is performed a step height is generated before and after the pixel position where the line shifting processing is performed in the main-scanning direction. The smoothing processing determines a line shifting point by using the correction information to smooth the step height.
Printer engines (also referred to as “printer units” or “image forming units”) have different maximum widths (main-scanning widths) in the main-scanning direction for forming the latent image by exposure scanning depending on a printer type. For example, a maximum width of the printer engine corresponds to an A3 size sheet or an A4 size sheet. The correction information is conventionally generated only for the main-scanning width of the printer engine by a soft module (correction information generation logic) which generates the correction information and the generated correction information can be used for the skew correction.
Out of the skew correction, the smoothing processing performs smoothing by determining whether there is a line shifting point based on the set correction information, to displace the centroid of the image data by less than a pixel unit. Therefore, depending on whether there is the line shifting point near an end portion of the main-scanning width of the printer engine, a correction result acquired by the skew correction varies. In other words, when the set correction information indicates that there is no line shifting point near the end portion of the main-scanning width where the skew of the exposure scanning line is to be offset by the smoothing processing, the smoothing processing is not performed and thus the appropriate skew correction is not performed.
As described above, when the generated correction information does not include the correction information about a wider area than the main-scanning width of the printer engine, the skew of the exposure scanning line cannot be sufficiently corrected near the end portion of the main-scanning width thereof.